Tubular reactors have become established over the past 70 years as a means of producing low and medium density polyethylene and ethylene copolymers. Such tubular reactors are large scale installations and typically operate at a pressure in excess of 2000 bar and sometimes as high as 3100 bar. It is believed that no other large scale industrial production process operates at a higher pressure.
It is well known that the economics of low density polyethylene manufacture strongly favour operating at a large scale and, therefore, there has been a long standing desire to develop tubular reactors of higher capacity. However, the operation of tubular polymerization reactors is subject to special considerations. Firstly, the extreme pressures must be handled in a safe manner. Secondly, commercial viability requires that the reactor operates as efficiently as possible with respect to the energy consumed and with respect to the conversion of monomer into polymer, with a minimum of downtime over a lifetime, which is typically decades. Unplanned downtime caused by equipment failure is particularly undesirable because the ethylene feed typically comes direct from a cracker which cannot be shut down without incurring very high cost, and so the ethylene must either be diverted under special arrangements with consequent disruption, if possible, or flared off with a consequent cost and waste of resources.
The demands placed on equipment, especially the compressors, increase as the scale of operation is increased, thereby making it more difficult to achieve the optimal operation required for economic viability. Moreover, because the polymerization process is strongly exothermic, the rate at which a reactor facility can produce polymer is limited by the rate at which heat can be removed, and therefore an increase in capacity can lead to problems associated with heat removal.
The above-mentioned factors, coupled with the difficulty of doing meaningful trials on a small scale, mean that any new design of reactor facility involves considerable challenges, which increase as the planned scale of the facility increases. For that reason, amongst others, for a long period of time activity has concentrated on the incremental increase in capacity of existing tubular reactors by debottlenecking, and there was no attempt to build new facilities with nominal capacities greater than those already proven, say, of 300 kilotonnes per annum (ktpa) or more, despite the economic benefits of operating at a larger scale.
Recently, however, a tubular reactor for polyethylene production of a nominal capacity of around 320 ktpa has been built and commissioned. It is believed, however, that the facility has experienced problems leading to undesirably high downtime. Accordingly, there remains a need for a design of a production facility and process for the manufacture of polyethylene and ethylene copolymers in a tubular reactor operating at high efficiency and reliability on a scale of greater than 300 ktpa.